Whilst NAC does convert to glutathione and could be why NAC has its benefits, both antioxidants play important roles in your body
It thus provides good information about the supply
Cells 10 , 540 (2021)
While serum glutathione tests exist, they tend to be expensive and highly specialized
However, clinical practitioners often recommend a "start low and go slow" approach, particularly for patients with severe chronic illness who may be highly sensitive to new supplements or prone to detoxification reactions (Herxheimer reactions)

Ingredients: Water, Centella Asiatica Extract (22.7%), Glycerin, Cetyl Ethylhexanoate, Butylene Glycol, Polyglyceryl-3 Methylglucose Distearate, Niacinamide (20,000ppm), Stearic Acid, Sodium Polyacryloyldimethyl Taurate, Silica, Sodium Hyaluronate, Glyceryl Stearate, Beeswax, Butyrospermum Parkii (Shea) Macad amia Ternifolia Seed Oil, Hydrogenated Polydecene, Betaine, Glutathione (1,000ppm), Trehalose, Adenosine, Citric Acid, Trideceth-10, Disodium Edta, Sodium Citrate, Caprylic/capric Triglyceride, Hydrogenated Lecithin, Cholesterol, Xanthan Gum, Nelumbo Nucifera Extract, Artemisia Annua Extract, Oryza Sativa (Rice) Extract, Saccharomyces Ferment, Solanum Melongena (Eggplant) Fruit Extract, Melaleuca Alternifolia (Tea Tree) Leaf Extract, Hydrolyzed Collagen, Squalane, Beta-glucan, Dipropylene Glycol, Hippophae Rhamnoides Fruit Extract, Madecassic Acid, Asiaticoside, Asiatic Acid, Propanediol, Hydroxyacetophenone, 1,2-hexanediol, Ethylhexylglycer in, Dipotassium Glycyrrhizate